As the early outcomes of human organ transplantation have dramatically improved, research focus has shifted toward solving the remaining problems associated with chronic immunosuppression. Calcineurin inhibitors remain the mainstay of immunosuppression in organ transplantation, but are associated with important side effects such as infections, diabetes, hypertension, nephrotoxicity and malignancy which can influence quality of life and survival rates. The ultimate goal of organ transplantation is to achieve clinical tolerance which is defined as stable normal graft function in the absence of immunosuppression.